Supporting the MPI_PROBE and MPI_IPROBE functions
does not require special packet transfers. The protocol is purely local
between the process and the agent. IMPI defines the conditions under
which a message is considered to be available for the purpose of probing.

Packet transfers are considered atomic operations, independent of the
medium's transfer mechanism. A message is considered available to the
destination process after its first packet (its only packet for short
messages) has been completely read by the agent, including the packet's
user data segment. The total length of the message is available in the
packet's pk_msglen field. Thus the status information needed by
the probe calls is available.